The manipulation of input and output light signals to and from optical fiber transmission lines generally requires that the signals be processed in some fashion, examples of which might include amplification, power splitting or the addition and/or dropping of signals. With the persistent trend towards miniaturization and integration, the optical circuits which best serve these processing functions are more and more being integrated on optical chips as a single module. The resulting optical circuits, which carry channel waveguides as their fundamental light-guiding elements, are generally referred to as planar lightwave circuits or PLCs. Current planar waveguide technology typically prepares a PLC by lithographically patterning light-guiding channels either directly upon (or buried beneath) the surface of a rigid planar substrate, or within a sequence of dielectric films separately deposited on the substrate.
In cases where the waveguide channels are formed in direct association with the substrate the substrate composition is usually chosen with a view to taking advantage of its specific electronic or electro-optic properties in addition to its mechanical characteristics. Patterning can be induced by ion exchange or by metallic diffusion. As an example of the latter process, a metallic film that has already been lithographically formed into a channel pattern can be heated to a temperature sufficient to induce a thermal diffusion of metal atoms into the surface region of the substrate (e.g. U.S. Pat. No. 5,749,132 by A. Mahapatra and S. A. Narayanan). In this manner a high-refractive-index light-guiding waveguide pattern can be created close to the surface of the substrate. The guides so formed can then be buried, if desired, by utilizing a second thermal-diffusion patterning process employing a metal that is able to generate a lower-refractive-index covering.
In PLCs where the waveguide channels are formed within a sequence of dielectric films deposited on a rigid substrate, the substrate usually plays only a thermal-mechanical roll. For these structures, the simplest situation sees the deposition of a sequence of three films (often referred to respectively as lower cladding (or buffer), core, and upper cladding) utilizing photolithography to pattern the required waveguide and component designs into the core layer. The refractive index of the core composition is chosen to be larger than those of the cladding layers to ensure good optical confinement within the core waveguides. An exposition of this general technology can be found in U.S. Pat. No. 4,902,086 by C. H. Henry et al.
In the context of the present invention the term ‘planar lightwave circuit’ (or PLC) should be interpreted to embrace all light-guiding circuits patterned into or onto rigid planar substrates. In particular, it should not be construed as limited to the specific categories examples of which have been described above.
In addition to signal processing circuits, which comprise optical network nodes, network termination points, such as light transmitters and light receivers, can also be integrated with other elements on a single PLC chip. Examples of light transmitters that can be so integrated are heterostructure end-emitting lasers, vertical cavity surface-emitting lasers and light emitting diodes. The most commonly used integrated light receivers are different types of photodiodes. For both transmitters and receivers, it is necessary that they be coupled to a single well-defined set of optical modes in the planar lightwave circuit. Generally the optical modes that carry the light signal around the optical chip are confined modes guided by waveguide channels. However, there are other unconfined optical modes (often designated as ‘radiation modes’ or ‘stray light’) present in the chip. These can enter devices on the chip and severely limit their performance. It therefore becomes necessary to design features that are capable of significantly limiting the undesirable access unconfined modes to these devices.
Light power in such radiation modes is almost exclusively scattered light, and there are usually several sources of scattered light on the optical chip. For example, there can be substantial power present on an optical chip in radiation modes due to imperfect coupling of a fiber to a planar waveguide at the chip interface. While it is disadvantageous to couple any power from a light signal into radiation modes on the chip, this is difficult to avoid in practice. Signal processing devices on the PLC chip can also be sources of scattered light. These sources are either there by design, such as in some types of variable optical attenuators where redundant light is dumped into the cladding, or they occur because of fabrication imperfections. On active chips, such as in waveguide amplifiers, another source of scattered light is amplified spontaneous emission from the gain material deposited on the chip. At the receiver end of the optical network, scattered light from any the above sources may interfere with the light signal propagating in a waveguide and can cause major signal degradation.
It therefore becomes necessary to devise a structure that can be used for optical isolation. Such a structure focuses on isolating a specific PLC device (such as the receiver) from light in radiation modes, but it can also be structured to isolate individual sources of scattered light from the rest of the chip. One common method of isolation in this context is the use of deep air trenches, geometrically positioned in a manner that can optimally intercept stray light that is propagating substantially parallel to a waveguide and redirect it away from the sensitive locations (see, for example, Pat. No. WO02097491 by D. Kitcher et al.). Another method is the introduction of light-absorbing regions to severely attenuate, rather than redirect, problematic radiation modes. In addition to the careful positioning of absorbing regions, the efficiency of stray light capture can be improved by decorating their shapes with protruding or notched facets to facilitate a more efficient coupling of scattered light into these lossy regions (see, for example, Pat. Nos. EP0883000 by T. S. Hoekstra, and WO03007034 by I. E. Day et al.). The structure by Day et al. is illustrated in FIG. 1.a, and comprises a waveguide 101, and light absorbing doped regions 102. The above references are directed solely to intercept stray light that propagates substantially parallel to the waveguide and therefore they rely on the proximity of the absorbing regions to the waveguide for efficient stray light absorption. Another approach to optical isolation is a monomode spatial optical filter (U.S. Pat. No. 5,093,884 by Gidon et al.), which is illustrated in FIG. 1.b), and comprises a curved waveguide section 103 and a light absorber 104 with an irregular sawtooth pattern and a geometrically asymmetrical shape with respect to the waveguide axis.